It is not possible to fly as such in Hello Neighbor. There is a way to jump in the air to great heights so that we can get an aerial perspective of everything, but not to fly as birds would do (or as was done in boomer shooters by entering a command in the console for game developers).
To "fly" in Hello Neighbor, you need to pick up the trash can in front of the house. We can take the bucket wherever we want and put it on the ground, then we climb on top of it. By jumping on it, we will see that we can gain a lot of height. Note, however, that this only works vertically: if we try to maneuver to the left or right, we will see that it does not work.
Thanks to this height, it is possible to enter the third floor, virtually inaccessible otherwise. However, doing so is not easy: you have to jump over the bucket to get as much height as possible, before climbing too high, you have to grab the bucket with your hands and finally, you have to try to access the third floor through the courtyard or the roof. This is a technique available to only a few, like killing the final boss in Ninja Gaiden to prevent it from causing damage to the player: it is only available to very skilled players.
Like many of the things that happen in Hello Neighbor, this trick is hardly deducible by the user, unless they are so desperate that they want to exhaust all possibilities or resort to trick compilations like this one you are reading right now. Of course, taking into account the absurd solutions of the adventure games of the '80s and '90s from where this game is a direct descendant, it fits perfectly.